This invention relates to transmitters which include frequency shift key modulation.
FSK (Frequency Shift Key) modulators are employed in optical data transmission to produce different frequencies corresponding to a binary data input stream. Most existing modulators employ a varactor diode to vary the capacitance of an oscillator circuit and, therefore, the output frequency. The problem with such a technique is that the capacitance vs. voltage characteristic of the varactor tends to vary from part to part and over temperature. Alternatively, a modulator may employ switching between two crystal oscillators, but this tends to produce discontinuous phase transitions in the output.
Recently, it has been proposed in an article identification system to utilize an FSK modulator which switches a capacitor or inductor into and out of an oscillator circuit in order to vary the frequency output (see U.S. Pat. No. 4,918,296 issued to Fujisaka et al). The switches employed were bipolar transistors. Use of such transistors would not be advantageous at the high frequencies required for the present application due to the relatively high and non-linear rf resistance the transistor would exhibit in the saturated state. Also, the oscillator signal amplitude would need to be small to avoid conduction of the collector-base junction of the transistor in the off-state.
A further problem with present modulators used in optical transmission involves producing a square wave output signal with a fixed duty cycle (usually higher than 50 percent) suitable for driving an optical device. High speed comparators could be used, but would tend to draw excessive power. Recent proposals have included the use of an inverter to produce the square wave, but generally without any means for fixing the duty cycle in spite of changes in the inverter threshold (see, e.g., U.S. Pat. No. 4,716,383 issued to Lofgren et al, U.S. Pat. No. 4,174,534 issued to Kotlarewsky, and U.S. Pat. No. 4,603,301 issued to Dukes, et al).